Swell compensator systems or wire tensioners on a trailing suction hopper dredger or other applications operate to reduce or eliminate overloadings which may be caused by swell due to heave motions of a wave. A typical swell compensator system is shown in FIG. 1 and uses a hydraulic system connected between a winch 12 and a load 14 it is carrying. The hydraulic system consists of a hydraulic cylinder 16 that keeps the wire rope 18 between winch 12 and load 14 taut. Hydraulic cylinder 16 is in fluid communication with a pressure vessel 20 that, in addition to the hydraulic fluid, contains air that acts as a gas spring. The hydraulic cylinder 16 serves to compensate for the motion of the waves, keeping the load 14 steady, and in the case of a trailing suction hopper dredger in FIG. 1, keeping the draghead in the correct position for a dredging operation. In some cases, a pressure vessel with medium separator between the hydraulic fluid and gas is used. When a medium separator is used, the gas used can be, for example, nitrogen instead of air.
If the wire 18 in the system breaks, cylinder 16 in the hydraulic system will extend at high speed due to the gas pressure, and this can damage cylinder 16 and/or connected equipment. Past systems dealt with this in two basic ways. The first way was using a high speed (proportional) valve 22 to block flow to the cylinder. However, such a system poses high demands on the components and the control system. The detection of wire failure and acting on it must happen very quickly, in a matter of milliseconds, for such a system to be effective. Additionally, blocking flow to cylinder 16 does not necessarily stop the movement of cylinder 16. Cylinder 16 may simply draw a vacuum die to the inertia of the rod and sheave block. The second option is to use a cushioning device on the rod side of the system. This is an inherent safety measure, but it only works properly if there is absolutely no air in cylinder 16, which may be hard to achieve, especially in systems without a medium separator.